1. Field of the Invention
The present invention relates to an image pickup method and an image pickup apparatus capable of broadening an apparent dynamic range of a video signal.
2. Related Background Art
Image pickup devices are used widely as an image pickup unit of VTRs with a built-in camera, still video cameras and the like. An image pickup apparatus using an image pickup tube of a video camera or a solid image pickup element has a dynamic range narrower than that of silver-salt photographing systems, and is therefore associated with some problems of “crushed white” with lost tonal levels in a bright portion and “crushed black” with lost tonal levels in a dark portion, under rear light or in other cases.
If such a case occurs with, for example, a video camera, a user increases an exposure amount of a main object with “crushed black” by manually controlling the iris or operating upon a rear light correction button to open the iris by about two steps.
Even if a proper exposure amount is obtained for the main object through rear light correction, the background has “crushed white” and becomes only white. Therefore, with a conventional image pickup apparatus, proper adjustment only of the exposure amount for a main object does not solve a problem of a narrow dynamic range of the image pickup apparatus.
In order to solve the problem of “crushed white” and “crushed black”, in an image pickup apparatus which uses a line scanner or the like for the conversion of a still image into electrical signals, a single image is synthesized from a plurality of images of the same object picked up at different exposure amounts.
In the case of such an image pickup apparatus taking only a still image, the positional relation between an object and the apparatus is fixed. Therefore, there is no shift between images although they are picked up at different timings, so that there is no practical problem even if a single image is synthesized from a plurality of images.
However, in the case of an image pickup apparatus such as a video camera or a still video camera, the above-described image pickup method cannot be applied reliably because the positional relation between an object and the apparatus may change minutely because of hand vibration or the like.
In such a case, the position of each of a plurality of images of the same object picked up at different timings has a small shift. Therefore, a single image synthesized from these images has the object with an image shift such as double or triple objects. This becomes a serious issue for recent compact and light video cameras which are likely to have hand vibration.